1. Field of the Invention
The present invention relates to an optical displacement gage, and more specifically, relates to improvements of an optical displacement gage for measuring a displacement amount of an object to be measured by utilizing interference between reflection light reflected from a reference surface and reflection light reflected from the object to be measured.
2. Description of the Related Art
Conventionally, there is known a film thickness measuring apparatus for measuring thickness of a transparent film by irradiating the transparent film with wide-band light containing various wavelengths as sensing light for measurement, and by dispersing the interference light of reflection light reflected on the front surface of the transparent film and reflection light reflected on the back surface of the same (see, for example, JP-A-7-4921 and JP-A-9-119815). Usually, in the film thickness measuring apparatus, the interference light including reflection light from the front surface of the transparent film and reflection light from the back surface of the same is dispersed by a dispersing element such as a diffraction grating, a prism or the like, and the dispersed light enters an image sensor having a plurality of light receiving elements arranged linearly. The film thickness is computed based on light reception amount data of individual elements sensed by the image sensor.
The film thickness measuring apparatus described in JP-A-7-4921 extracts neighboring peak points from the intensity distribution of the interference light after the dispersion with respect to the wavelength and computes the film thickness from a wave number difference between the peak points. In addition, the film thickness measuring apparatus described in JP-A-9-119815 performs Fourier transformation of the intensity distribution of the interference light after the dispersion with respect to the wavelength, and determines the peak center from a phase gradient of each frequency component so as to compute the film thickness.
As such a measuring apparatus utilizing interference of reflection light, there is also known an optical displacement gage for measuring a displacement amount of an object to be measured utilizing interference between reflection light from a reference surface and reflection light from the object to be measured. This optical displacement gage includes two types. One type performs Fourier transformation of optical intensity distribution with respect to the wave number of the interference light after the dispersion so as to determine optical intensity distribution with respect to spatial frequency of the light intensity for the wave number, and decides the displacement amount from the spatial frequency at the local maximum. The other type decides the displacement amount from phase of frequency component corresponding to the spatial frequency at the local maximum in the optical intensity distribution with respect to the wave number.
The optical displacement gage that decides the displacement amount directly from the spatial frequency at the local maximum can measure the displacement amount even if the distance between the object to be measured and the reference surface is relatively long. However, if the surface of the object to be measured is dirty with fingerprints or the like, there is a problem that intensity distribution of the interference light will cause a distortion, which affects measurement accuracy to decrease. On the other hand, the optical displacement gage that decides the displacement amount from phase of frequency component corresponding to the spatial frequency at the local maximum is hardly affected by dirt on the surface of the object to be measured, so its measurement accuracy is high compared with the type that decides the displacement amount from the spatial frequency. However, this type has a problem that a measurable range is narrow because it can decide the phase only in the range of 360 degrees.
Further, in case of the film thickness measuring apparatus using a light source device such as a halogen lamp capable of generating white light having a wide wavelength bandwidth, the interference light is transmitted to a spectroscope usually by using an optical fiber having relatively large diameter for sustaining the light amount at a predetermined level or higher. In this measuring apparatus, the sensing light has a wide wavelength bandwidth so that the light amount for each wavelength is not sufficient. Therefore, there is a problem that wavelength resolution of the spectroscope cannot be increased up to a predetermined level or higher.